∀ a0. a0 = a ∨ a0 ∈ s ↔ a0 = b ∨ a0 ∈ s0 ⊦ a = b, a ∈ s0
Used simplifier rules: Extension: ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2);%In-insert: ⊦ a ∈ s ++ b ↔ a = b ∨ a ∈ s;